The present invention relates generally to fuel control systems for internal combustion engines, and particularly to such a system using air-fuel mixing control valves operable to open in response to the width of feedback correction pulses derived from a sensed air-fuel ratio within the exhaust system, wherein the width of the pulses is controlled within the effective working range of the control valves. In a closed loop fuel control system for internal combustion engines, the deviation of the air fuel ratio within the exhaust system from a desired value is detected to derive a feedback correction signal which is used to modulate the width of electrical pulses. The electrical pulses are supplied to air-fuel mixing control valves to control the air-fuel ratio. However, the control valve is incapable of responding to rapidly interrupted signals. For example, the conventional air-fuel mixing control valves have a minimum valve opening time of 5 milliseconds and a minimum valve closure time of 2.5 milliseconds. If the feedback control signal varies excessively in amplitude so that the width of the feedback correction pulses becomes smaller than the 5-milliseconds duration or the interval between successive pulses becomes smaller than the 2.5-millisecond duration, the corrective action cannot be faithfully reflected in the controlled air-fuel ratio.
Practically, the electrical control pulses are generated by a comparator which compares the amplitude of the feedback correction analog signal with the instantaneous value of triangular wave pulses and provides an output at one of two discrete voltage levels depending upon the relative magnitude of the input voltages. If the feedback analog signal represents an integration of the deviation of the air-fuel ratio in the exhaust system, the analog signal may build up above the maximum amplitude of the triangular wave so that the output of the comparator remains at one of the discrete voltage levels and control valves remain open (or closed) over time. Therefore, the corrective action on the air-fuel ratio cannot be faithfully effected due to the building-up of the integration as well as to the limited working range of the control valve. Furthermore, the control valves tend to hesitate when the control pulses is reapplied.